This invention relates to a method and apparatus for detecting an absolute position of a rod member used for a positioning system in a machine such as a machine tool and a die casting machine.
FIG. 1 shows a schematic diagram of a position detecting means as the most important portion for a positioning system of a machine such as a machine tool and a die casting machine. Referring to FIG. 1, a plurality of magnetic materials 2 and the non-magnetic materials 3 are formed or arranged coaxially and alternatingly with equal pitches P on the outer or inner peripheral surface of a movable cylindrical rod 1. Primary excitation coils 6A and 6B and secondary induction coils 5A and 5B are coaxially located respectively in magnetic shield cores 10A and 10B which are located in a cylindrical detecting head portion 4 stationary and arranged so as to surround the rod 1. When the primary coils 6A and 6B are excited with an alternating current provided with phases I.multidot.sin .omega.t and I.multidot.cos .omega.t, respectively, the magnetic resistance is changed every one pitch in accordance with the movement of the rod 1 in the axial direction by the alternating arrangement of the magnetic materials 2 and the non-magnetic materials 3. The change of the magnetic resistance results in an induced potential change to the secondary induction coils 5A and 5B and an induced electromotive force E is expressed as follows. ##EQU1## where K: Conversin coefficient,
X: Moving distance of rod, PA1 P: Pitch
In view of the phase ##EQU2## in this equation (1) it is found that only the moving distance X of the rod 1 is phase-shifted in comparison with the phase I.multidot.sin .omega.t in the primary excitation. Accordingly, the moving distance X is obtained by detecting the phase difference between these phases and taking out the phase difference as signal EO representing an electric potential.
According to the unit shown in FIG. 1, however, the absolute distance can be obtained only in a case where the moving distance X of the rod 1 is within the range of one pitch P thereof. Namely, since the moving distance X is obtained only by an incremental form, when the moving distance X is over one pitch P, a quasi-absolute distance can be obtained by integrating the outputted potential signal EO, FIG. 2 shows a variation of the potential signal EO and the horizontal axis of the graph shown in FIG. 2 represents the moving distance X and the vertical axis thereof represents the level of the potential signal EO, which is outputted repeatedly every one pitch. The absolute position is thus detected by integrating the outputted potentials; but at the starting time of the movement of the rod, the integrated amount has been reset, so that the absolute position can not be detected. For this reason, in the conventional technique, it is restricted that the rod 1 should be absolutely returned to the original reference position at the starting time of the rod movement.
FIG. 3 shows another example of a device for detecting the absolute position without having the restriction described above. In this device, parallelly arranged rods 1A and 1B are integrated so as to be both movable in their longitudinal, i.e. axial directions and detecting heads 4A and 4B are located near the rods 1A and 1B, respectively. The rod 1A is provided with a plurality of magnetic materials 2A and non-magnetic materials 3A alternatingly and the rod 1B is also provided with a plurality of the magnetic materials and the non-magnetic materials 3B. In this example, the rods 1A and 1B are constructed so that the pitch PA between one pair of the materials 2A and 3A is different from the pitch PB between one pair of the materials 2B and 3B, and accordingly the detecting heads 4A and 4B detect different electric potentials EO1 and EO2 as shown in FIGS. 4A and 4B when the rods 1A and 1B are moved together. The absolute position can be detected by the combination of the potentials EO1 and EO2. In this device, however, it is required to locate the two rods 1A and 1B and the two detecing heads 4A and 4B, and this arrangement makes the whole structure of the device large, thus being expensive and not economical.